Introduction to Circadian Rhythms

Abstract

One of the most obvious adaptive features of life on earth is the ability of almost all species to change their behavior on a daily or 24-hour basis (Aschoff, 1981). A remarkable feature of the daily rhythms that are observed in organisms as diverse as algae, fruit flies, and humans is that they are not simply a response to the 24-hour changes in the physical environment imposed by the principles of celestial mechanics, but instead arise from an internal time-keeping system. This time-keeping system, or biological clock(s), allows the organism to predict and prepare in advance for the changes in the physical environment that are associated with night and day. Thus, the organism adapts, both behaviorally and physiologically, to meet the challenges associated with the daily changes in the external environment, and there is temporal synchronization between the organism and the external environment. Of equal, but perhaps less appreciated, importance is the fact that this biological clock, like a conductor of a symphony orchestra, provides internal temporal organization and insures that internal changes take place in coordination with one another. Just as living organisms are organized spatially, they are also organized temporally to insure that there is “internal synchronization” between the myriad biochemical and physiological systems in the body (Pittendrigh, 1993).